Weddingbee boards

Just be careful about how many stamps it takes to make the amount of postage you need.  Remember that whether you hand cancel or not, there has to be ink on every stamp - that means at least one thwack of the cancellation inkstamp for every two stamps - and sometimes, if you have a postal employee hand cancel them for you, one for each stamp.  I kid you not, I have seen this.  The envelopes that look so fantastic in Martha's photos with the row or array of vintage stamps across the top will be an unholy mess by the time your guests see them.

That said, eBay is probably the best place to get them.  Most of the other sites seem to cater mostly to people looking for collectible stamps, which means that they charge a lot more.  Whereas on eBay you can always find somebody selling their grandma's stamp collection, without any regard for what they are actually worth.  And it's pretty easy to search by postage value, or by an actual description of the stamp, as long as you know what you're looking for.